Turning to FIG. 1, an example of a conventional Automatic Gain Control (AGC) circuit 100 can be seen. While the AGC circuit 100 is generally referred to having the characteristic of “gain control,” it is actually an Automatic Level Control (ALC) circuit. In operation, the level detector 104 detects the level of the output signal OUT (usually peak level), and the controller 106 compares the level of the output signal OUT against a fixed (reference) level REF. Based on the results of this comparison, the controller 106 can then adjust, with control signal CNTL, the gain of the Variable Gain Amplifier (VGA) to generally maintain level of the output signal OUT. When the AGC circuit 100 is used in a linear signal conditioning device (such as a linear equalizer), the AGC circuit 100 is non-transparent to link training (i.e., the level of the output signal OUT does not respond to level changes in the input signal IN during link training) and may prevent optimal system configuration in terms of bit error rate, power, and electromagnetic interference (EMI). Additionally, limiting (non-linear) devices can distort the transmitter waveform and make it difficult for the receiver Decision Feedback Equalizer (DFE) to recover the data correctly. Thus, there is a need for an improved AGC circuit.
Some examples of conventional system are: U.S. Pat. No. 6,415,003; U.S. Pat. No. 6,421,381; U.S. Pat. No. 7,050,517; U.S. Patent Pre-Grant Publ. No. 2003/0016770; and Hidaka et al., “A 4-Channel 1.25-10.3 Gb/s Backplane Transceiver Macro With 35 dB Equalizer and Sign-Based Zero-Forcing Adaptive Control,” IEEE J. of Solid-State Circuits, Vol. 44, No. 12, December 2009.